On some of our projects the Coanda screen blocks up at certain times of the year (typically autumn). On one site in particular the screen can almost completely block within hours of being cleaned. Has anyone else found this? any ideas for possible cause or cure?

My experience is strangely mixed on very similar sites. One site blocks as you say, within an hour, the other on the same watercourse, but a mile upstream, stays clear. The difference is only a reservoir between two, in which very fine weed seems to grow. The bulk of the big stuff is same across both intakes - leaves, weed etc., but the fine weed forms a blanket over the screen. I never found a solution with the coandas, so have swapped to 6mm gap bar screens in problematic intake location and the screens stay much better cleaned. Interestingly this screen cost more than the coandas (just lots of metal in them) and the EA weren't that keen to allow the change, but it has sorted it. I guess your situation might be different, but there do appear to be various very site specific problems which seem to affect certain intakes. Other issues I see on occasion are certain types of laminar stones getting jammed in the screen gap where the catchment is similar to slate in make up, and also funny growth between screen bars which is hard to clear with bristles and which take pressure washing to shift. Neither of these things I really anticipated.

We have made some further progress on this particular site. There is just nothing visible to the naked eye on the screen when it blocks so we did some chemical analysis and it appears that the blocking is somewhat related to iron content in the water. The iron content spikes after drought periods and this coincides with times that the screen blocks.

We did find the problem.....turned out not to be the chemical analysis of the water but effectively an airlock forming under the screen. We were misled by the evidence because at commissioning there was no problem and the intake worked properly. The odd behaviour just happened to coincide with the high iron content and so we thought that this was the problem. In actual fact it is likely that there was a small air path to break the vacuum when the screen was new which perhaps has blocked with algae over time. Adding an extra vent to the screen chamber has solved the problem. Funny how evidence can be misleading!

I like the eventual simple solution to the problem, I wish they were all that easy. As a Materials Engineer by training you had my head spinning with the Fe analysis! I can now safely rely on science again phew!